Cut fish fillets into large chunks. Drain pineapple, reserving juice. Add water to juice to make 1-1/4 cups. Combine pineapple mixture, vinegar, brown sugar, cornstarch, soy sauce, salt, and garlic salt and blend well.
Saute fish in oil for 10 minutes or until fish is firm and white. Add liquid mixture and cook and stir 5 minutes or until sauce has thickened. Add water chestnuts, bell pepper, tomato and pineapple and cook until vegetables are heated through and fish flakes easily with fork.